<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <!-- 
        请用户输入年份后再输入月份，并判断这个月份有几天
    -->
</head>

<body>

</body>
<script>
    let year = parseInt(prompt('请输入年份:'));
    let month = parseInt(prompt('请输入月份'));
    alert(monthDay(year, month));

    //是闰年
    function isLeapYear(year) {
        return (year % 4 == 0 && year % 100 != 0 || year % 400 == 0);
    }

    function monthDay(year, month) {
        switch (month) {
            case 1:
            case 3:
            case 5:
            case 7:
            case 8:
            case 10:
            case 12:
                return 31;
                break;
            case 4:
            case 6:
            case 9:
            case 11:
                return 30;
                break;
            case 2:
                if (isLeapYear(year)) {
                    return 28;
                } else {
                    return 29;
                }
        }
    }
</script>

</html>